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Pressureless sintering and physical properties of ZrB2-based composites with ZrSi2 additive
Available online at www.sciencedirect.com
Scripta Materialia 58 (2008) 579–582 www.elsevier.com/locate/scriptamat
Pressureless sintering and physical properties of ZrB2-based composites with ZrSi2 additive Shu-Qi Guo,a,* Yutaka Kagawa,a,b Toshiyuki Nishimurac and Hidehiko Tanakac a
b
Composites and Coatings Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan c Nano Ceramic Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 31 October 2007; accepted 12 November 2007 Available online 20 December 2007
Fully dense ZrB2-based composites with ZrSi2 additive were pressureless-sintered at 1650 °C. The thermal and electrical conductivities of the resulting composites were examined. The effects of ZrSi2 amount on the sinterability and the conductivities of the composites were assessed. The thermal conductivities observed for all of compositions were in the range 74.16–107.16 W (m K)1. The electrical conductivities observed for all of compositions were in the range 0.96 105–1.19 105 (X cm)1. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Zirconium diboride; Zirconium disilicide; Pressureless sintering; Thermal conductivity; Electrical conductivity
Zirconium diborides (ZrB2)-based composites have extremely high melting points (>3000 °C), high thermal and electrical conductivities, chemical inertness against molten metals, and great thermal shock resistance [1,2]. These unique mechanical and physical properties have never been achieved by other ceramics materials. Thus, the ZrB2-based composites have become an important class of materials for structural applications at ultra-high temperatures of above 1800 °C. However, the densification of ZrB2 powder generally requires very high temperatures (>2100 °C) and external pressure because of covalent bond and low self-diffusivity [3]. To improve sinterability, nitrides, like AlN, Si3N4, ZrN, are added to pure ZrB2 [4–6], producing an intergranular liquid phase that aids the densification of ZrB2. Even for the ZrB2-based ceramics with nitrides, however, a sintering temperature of above 1900 °C is needed for obtaining near-fully dense ceramics. An alternative approach is to add disilicide of transition metals for further lowering sintering temperature. Recently, MoSi2containing ZrB2 ceramics have been developed. Near fully dense ZrB2-based composites with MoSi2 were hot-pressed at 1750 °C and/or 1800 °C [7,8]. Also, this composition was successfully pressureless-sintered at 1850 °C [9]. This suggests that the disilicide of transition
* Corresponding author. Tel.: +81 0 29 859 2223; fax: +81 0 29 859 2401; e-mail: [email protected]
metals is a potential effective additive for ZrB2 ceramics. Therefore, it is necessary for developing low temperature pressureless sintering of ZrB2 ceramics to explore a new disilicide additive of transition metals. In the present study, ZrB2-based composites with ZrSi2 were pressureless-sintered at 1600 °C and 1650 °C. The thermal and electrical properties in the composites were investigated at room temperature by a nanoflash technique and a current–voltage method, respectively. Also, the effects of ZrSi2 amount on the densification, thermal, and electrical conductivities were examined. The starting powders used in this study were: ZrB2 powder (Grade F, Japan New Metals, Tokyo), average particle size 2.1 lm, ZrSi2 powder (Grade F, Japan New Metals), average particle size 2.5 lm. The ZrB2 powder contains 1.6 wt.% B2O3, and the ZrSi2 powder contains 1.4 wt.% SiO2. In order to examine effects of ZrSi2 amount on the densification and properties, four series of ZrB2–ZrSi2 compositions were prepared in this study. Hereafter, four series of ZrB2–ZrSi2 compositions are denoted as ZSZ-1, ZSZ-2, ZSZ-3, and ZSZ-4, respectively. The detailed compositions are shown in Table 1. The powder mixtures were wet-mixed using SiC milling media and ethanol for 24 h, and the resulting slurries were then dried. Before sintering, the dried mixtures were sieved through a metallic sieve with –60-mesh screen size. Pellets were prepared by uniaxial pressing, following by cold isostatic pressing under 350 MPa. Then, the
1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.11.019
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S.-Q. Guo et al. / Scripta Materialia 58 (2008) 579–582
Table 1. Compositions, densities, shear modulus, Young’s modulus, and Poisson’s ratio of the ZrB2–ZrSi2 composites pressureless-sintered at 1650 °C for 60 min
ZSZ-1 ZSZ-2 ZSZ-3 ZSZ-4
Compositions (vol.%) ZrB2
ZrSi2
90 80 70 60
10 20 30 40
Theoretical density (g/cm3)
5.97 5.85 5.73 5.61
True density (g/cm3)
5.77 5.82 5.71 5.60
pellets were pressureless-sintered (FVHP-1-3, Fuji Electric Co. Ltd., Tokyo, Japan) in a graphite crucible at 1600 °C and 1650 °C for 60 min under vacuum of 7.0 103 Pa. After loading the pellets into the crucible, the furnace was heated at a heating rate of 30 °C min1 to the sintering temperature. The resulting pressureless-sintered sample was approximately 10 mm in diameter and 3 mm in thickness. The final densities, were measured by the Archimedes method with distilled water as medium. The elastic moduli measurements of the composites were determined using ultrasonic equipment. The details of measurements and calculations were reported elsewhere [10]. The obtained densities, shear modulus, G, Young’s modulus, E, and Poisson’s ratio, m, of the composite were also summarized in Table 1. The shear modulus and Young’s modulus lowered with increasing ZrSi2 amount because of the lower moduli of ZrSi2 phase than those of ZrB2 one. One exception was a lower shear modulus and Young’s modulus for the ZSZ-1 than those of the ZSZ-2 and ZSZ-3, as a result of the presence of pores (4 vol.%) because the porosity led to the decreases of both the moduli [10]. X-ray diffraction (XRD) was used for crystalline phase identification of the composites. Microstructure of the resulting composites was observed by field emission scanning electron microscopy (FE-SEM). The thermal diffusivity, a, of the composites was measured on a disk-shaped specimen with a diameter of 10 mm and thickness of 2 mm using the nanoflash technique (LFA447/2-4 N, Nanoflash, NETZSCH-Geratebau GmbH, Postfach, Germany). Prior to the measurements, the samples were coated with a colloidal graphite spray in order to enhance the absorption of the xenon light pulse energy and the emission of IR radiation to the temperature detector. Also, the heat capacity, Cp, was measured with alumina as the reference material. Subsequently, the thermal conductivity of the composites, km, is determined from thermal diffusivity, a, heat capacity, Cp, and density, q, of the composites according to the following equation [11]: k m ¼ qC p a
ð1Þ
The calculated values of the thermal conductivity were corrected for the porosity of the composites by the equation reported elsewhere [12]. Moreover, the electrical conductivity measurements of the composites were performed using the four-wire probe at room temperature. A powder supply (Model: 6220, Keithley, Cleveland, Ohio, USA) and digital multimeter (Model: 2182 Nanovoltmeter, Keithley) were used to measure the IV characteristics of the samples.
Relative density (%TD)
Elastic properties
95.7 99.5 99.7 99.9
G (GPa)
E (GPa)
m
179 192 193 164
405 443 449 383
0.13 0.15 0.17 0.17
In Figure 1, the relative densities of the four compositions ZrB2 materials pressureless-sintered at 1600 °C and 1650 °C for 60 min are presented. At 1600 °C, the relative density strongly depended on amount of ZrSi2, and the density increased with ZrSi2 addition. The relative density exceeding 90% was obtained for an additive amount equal to or greater than 30 vol.%. In particular, for 40 vol.% ZrSi2-containing ZrB2 composition (ZSZ4), the density of about 97.1% was obtained. At 1650 °C, the density exceeding 99% was obtained for ZSZ-2, ZSZ-3 and ZSZ-4 powders regardless of ZrSi2 amount. This means that the fully dense ZrB2-based composites with ZrSi2 may be pressureless-sintered at 1650 °C for an additive amount equal to or greater than 20 vol.%. Even for ZSZ-1 powder, the high relative density of 95.7% was obtained. It is evidence that the sinterability of ZrB2 ceramics is remarkably improved by ZrSi2 addition. This indicated that ZrSi2 is an effective additive for lowering the sintering temperature of ZrB2 ceramics. It is common for SiO2 and B2O3 films to be present on the surfaces of the starting ZrSi2 and ZrB2 powder, respectively. It is known that the SiO2 reacts with B2O3 to produce an intergranular liquid phase at a high temperature (below 1500 °C) [13]. Improvement of densification due to the presence of intergranular liquid phase is documented in the literature. Sciti et al. [14] showed that the addition of MoSi2 significantly improved densification of HfB2 powders, as a result of the presence of intergranular liquid phase. They concluded that the presence of intergranular liquid phase favors the process of grain rearrangement as well as improves the packing density of particles, and removes the oxide species from the surface of HfB2 particles. Similar causes are expected for the ZrB2-based composites with ZrSi2 additive investigated in this study.
105 t=60 min 100 Relative Density (%)
Materials
95 90 85 80
1600°C 1650°C
75 70
5
10
15
20
25
30
35
40
45
Amount of ZrSi2 (vol.%)
Figure 1. Plots of relative density measured for the pressurelesssintered ZrB2–ZrSi2 composites as a function of ZrSi2 amount.
S.-Q. Guo et al. / Scripta Materialia 58 (2008) 579–582
Intensity (a.u.)
ZrB 2
ZrSi 2
SiC
ZrO 2
(d)
(c) (b) (a)
20
30
40
50 60 2θ (degree)
70
80
Figure 2. X-ray diffraction patterns of the ZrB2–ZrSi2 composites pressureless-sintered at 1650 °C for 60 min: (a) ZSZ-1, (b) ZSZ-2, (c) ZSZ-3, and (d) ZSZ-4.
In Figure 2, X-ray diffraction patterns for the pressureless-sintered ZrB2–ZrSi2 composites are presented. From this figure, it is found that the ZrB2 and ZrSi2 phases are primary crystalline phases and the trace amount of ZrO2 and SiC phases are also present for all the compositions. The trace of ZrO2 should be attributed to both oxygen contamination of the starting ZrB2 powder and to oxygen take-up during the milling procedure. The presence of SiC phase should be associated with C contamination of the starting ZrSi2 powder as well as with SiC contamination of media during the milling procedure. Microstructure of the ZrB2–ZrSi2 composites is observed under backscattered electron FE-SEM imaging; typical examples are shown in Figure 3. From these images, it is found that the equiaxed ZrB2 grains (brighter contrast) were homogeneous in the microstructure of the composites. In contrast, the ZrSi2 grains (dark contrast) were inhomogeneous and had a bimodal grain size distribution. Some ZrSi2 grains were very fine and they presented at ZrB2 grain boundaries and/or within ZrB2 grains (indicated by arrows in Fig. 3b). Apparently, the fine ZrSi2 particulars result from the mixing process of powders. This means that the mixing process is effective in reducing the size of
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ZrSi2 particles. Some ZrSi2 grains were considerably coarse, and they presented at multiple grains junctions (indicated by arrows in Fig. 3c). The ZrSi2 grains were randomly and individually present in the isolated locations. For ZSZ-4 composition, however, the ZrSi2 agglomerate was observed (indicated by arrows in Fig. 3d). In addition, some pores were observed for ZSZ-1 composition (indicated by arrows in Fig. 3a), while only a little of pores were observed for ZSZ-2, ZSZ-3 and ZSZ-4 compositions. The measured heat capacities, thermal diffusivities, and thermal conductivities of the pressureless-sintered ZrB2–ZrSi2 composites are summarized in Table 2. Although the heat capacities lowered with ZrSi2 amount, this effect is considerably weak. On the other hand, the thermal diffusivity of the composites was almost the constant between 10 vol.% and 20 vol.% ZrSi2, and the diffusivity then decreased with increase of ZrSi2 amount. In particular, for ZSZ-4 composition, a significant decrease was observed. Furthermore, the decrease of the thermal diffusivity due to ZrSi2 addition is larger than that of the heat capacity. This means that the effect of ZrSi2 was larger for the thermal diffusivity than for the heat capacity. The obtained thermal conductivity of the composites significantly lowered with ZrSi2 amount, as a result of the lowered heat capacity as well as of the lowered thermal diffusivity. It is known that the thermal conductivity of the composites depended on the thermal conductivity of the components and the thermal resistance of the grain boundaries. The thermal conductivity of ZrB2 is higher than that of the ZrSi2 [15]. This implies that ZrSi2 additions should be lowered the thermal conductivity of ZrB2–ZrSi2 composites. These effects are closely linked to the amount of ZrSi2 because it affects the heat flow resistance through the components and the grain boundaries. In the studied ZrB2–ZrSi2 composites, the thermal conductivity significantly decreased with ZrSi2 amount (Table 2). This suggests that thermal conductivity of the composites strongly depended on the amount of ZrSi2. The measurements of thermal properties showed
Figure 3. Typical examples of backscattered electron FE-SEM images for the ZrB2–ZrSi2 composites pressureless-sintered at 1650 °C for 60 min: (a) ZSZ-1, (b) ZSZ-2, (c) ZSZ-3, and (d) ZSZ-4.
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Table 2. Thermal properties measured at room temperature in the ZrB2–ZrSi2 composites pressureless-sintered at 1650 °C for 60 min Materials
Heat capacity Cp (J (g K)1)
Thermal diffusivity a (mm2/s)
Measured thermal conductivity km (W(m K)1)
Corrected thermal conductivity kc (W(m K)1)
ZSZ-1 ZSZ-2 ZSZ-3 ZSZ-4
0.56 ± 0.01 0.53 ± 0.01 0.52 ± 0.01 0.50 ± 0.01
30.75 ± 0.31 30.96 ± 0.22 29.01 ± 0.21 26.43 ± 0.09
98.32 ± 0.89 95.83 ± 0.62 86.29 ± 0.58 74.01 ± 0.27
107.16 ± 0.89 96.79 ± 0.62 86.81 ± 0.58 74.16 ± 0.27
Table 3. Electrical properties measured at room temperature in the ZrB2–ZrSi2 composites pressureless-sintered at 1650 °C for 60 min Materials
Electrical resistivity R (X cm)
Electrical conductivity r (X cm)1
ZSZ-1 ZSZ-2 ZSZ-3 ZSZ-4
9.33 106 8.64 106 8.39 106 10.40 106
1.07 105 1.16 105 1.19 105 0.96 105
that the presence of ZrSi2 led to an increase of the heat flow resistance through the components and the grain boundaries (Table 2). The resistance of heat flow increased with increasing ZrSi2, as a result of the reduction of high conductivity ZrB2-phase and the increase of ZrB2/ZrSi2 and/or ZrSi2/ZrSi2 grain boundaries. Thus, accompanying the increase of ZrSi2, the lowered thermal conductivity probably resulted from the high thermal transport resistance of ZrSi2 phase as well as from a higher intergranular thermal resistance at the ZrB2/ ZrSi2 and/or ZrSi2/ZrSi2 grain boundaries than that at the ZrB2/ZrB2 grain boundary. The measured electrical resistivities and conductivities of the pressureless-sintered ZrB2–ZrSi2 composites are summarized in Table 3. It is found that the electrical conductivity of the composites was almost constant for an additive amount equal to or lower 30 vol.% and the conductivity lowered for 40 vol.% ZrSi2-containing ZrB2 ceramic (ZSZ-4). One exception is the low electrical conductivity for the additive amount of 10 vol.% (ZSZ-1) because of the presence of pores. This shows that the ZrSi2 addition has no affect on the electrical conductivity of the composites when the ZrSi2 amount was equal to or lower than 30 vol.% although the conductivity of ZrSi2 is lower that of ZrB2 [15]. This means that the electrical conductivity is insensitive to added ZrSi2 amount for an additive amount equal to or lower than 30 vol.%. In the studied composites, the ZrSi2 grains randomly and individually presented between the ZrB2 grains (Fig. 3). Thus, the ZrB2 grains still remained a closely contact each other for an additive amount equal to or lower than 30 vol.%. This means that the ZrB2 grains with a higher electrical conductivity in the composites formed a network-like structure for providing an electrical path with lower resistance. However, this structure is insufficient for retention of a high thermal conductivity. This seems to suggest that the interfacial resistance at grain boundaries is larger for
heat flow than for electron migration. For ZSZ-4 composition, however, the electrical conductivity of the composite decreased because of the presence of a large amount of ZrSi2 phase and the ZrSi2 particulars agglomerates. This suggests that 30 vol.% ZrSi2 is critical for the high electrical conductivity of ZrB2 ceramic to remain. In summary, the near-fully dense ZrB2-based composites with ZrSi2 additive were pressureless-sintered at 1650 °C for 60 min in vacuum for an additive amount equal to or greater than 20 vol.%. The relative densities exceeding 99% was obtained for these powders. The thermal conductivities of ZrB2–ZrSi2 composites lowered with ZrSi2 amount, and they were in the range 74.16 and 107.16 W (m K)1. On the other hand, the electrical conductivities of ZrB2–ZrSi2 composites were almost the constant for an additive amount equal to or lower than 30 vol.%. However, the conductivity significantly lowered as the ZrSi2 amount was 40 vol.%. The measured electrical conductivities were in the range 0.96 105 to 1.19 105 (X cm)1. [1] K. Kuwabara, Bull. Ceram. Soc. Jpn. 37 (2002) 267. [2] K. Upadhya, J.-M. Yang, W.P. Hoffmann, Am. Ceram. Soc. Bull. 76 (1997) 51. [3] M. Pastor, Metallic Borides: Preparation of Solid Bodies. Sintering Methods and Properties of Solid Bodies, in: V.I. Matkovich (Ed.), Boron and Refractory Borides, Springer, New York, 1977, pp. 457–493. [4] F. Monteverde, A. Bellosi, Scripta Mater. 46 (2002) 223. [5] F. Monteverde, A. Bellosi, Solid State Sci. 7 (2005) 622. [6] F. Monteverde, A. Bellosi, Adv. Eng. Mater. 5 (2003) 508. [7] A. Bellosi, F. Monteverde, D. Sciti, Int. J. Appl. Ceram. Technol. 3 (2006) 32. [8] S.Q. Guo, T. Nishimura, Y. Kagawa, H. Tanaka, J. Am. Ceram. Soc. 90 (2007) 2255. [9] D. Sciti, S. Guicciardi, A. Bellosi, G. Pezzotti, J. Am. Ceram. Soc. 89 (2006) 2320. [10] S.Q. Guo, N. Hirosaki, Y. Yamamoto, T. Nishimura, M. Mitomo, J. Euro. Ceram. Soc. 23 (2003) 537. [11] W.J. Parker, W.J. Jenkins, C.P. Butler, G.L. Abbott, J. App. Phys. 32 (1961) 1679. [12] W.D. Kingery, H.K. Bowen, D.R. Uhlman, Introduction to Ceramics, Eiley, New York, 1976, p. 636. [13] T.J. Rockett, W.R. Foster, J. Am. Ceram. Soc. 48 (1965) 75. [14] D. Sciti, L. Silvestroni, A. Bellosi, J. Mater. Res. 21 (2006) 1460. [15] Manufactures data, Japan New Metals Corporation Ltd., Tokyo, Japan, (http://www.jnm.co.jp).
Scripta Materialia 58 (2008) 579–582 www.elsevier.com/locate/scriptamat
Pressureless sintering and physical properties of ZrB2-based composites with ZrSi2 additive Shu-Qi Guo,a,* Yutaka Kagawa,a,b Toshiyuki Nishimurac and Hidehiko Tanakac a
b
Composites and Coatings Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan c Nano Ceramic Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 31 October 2007; accepted 12 November 2007 Available online 20 December 2007
Fully dense ZrB2-based composites with ZrSi2 additive were pressureless-sintered at 1650 °C. The thermal and electrical conductivities of the resulting composites were examined. The effects of ZrSi2 amount on the sinterability and the conductivities of the composites were assessed. The thermal conductivities observed for all of compositions were in the range 74.16–107.16 W (m K)1. The electrical conductivities observed for all of compositions were in the range 0.96 105–1.19 105 (X cm)1. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Zirconium diboride; Zirconium disilicide; Pressureless sintering; Thermal conductivity; Electrical conductivity
Zirconium diborides (ZrB2)-based composites have extremely high melting points (>3000 °C), high thermal and electrical conductivities, chemical inertness against molten metals, and great thermal shock resistance [1,2]. These unique mechanical and physical properties have never been achieved by other ceramics materials. Thus, the ZrB2-based composites have become an important class of materials for structural applications at ultra-high temperatures of above 1800 °C. However, the densification of ZrB2 powder generally requires very high temperatures (>2100 °C) and external pressure because of covalent bond and low self-diffusivity [3]. To improve sinterability, nitrides, like AlN, Si3N4, ZrN, are added to pure ZrB2 [4–6], producing an intergranular liquid phase that aids the densification of ZrB2. Even for the ZrB2-based ceramics with nitrides, however, a sintering temperature of above 1900 °C is needed for obtaining near-fully dense ceramics. An alternative approach is to add disilicide of transition metals for further lowering sintering temperature. Recently, MoSi2containing ZrB2 ceramics have been developed. Near fully dense ZrB2-based composites with MoSi2 were hot-pressed at 1750 °C and/or 1800 °C [7,8]. Also, this composition was successfully pressureless-sintered at 1850 °C [9]. This suggests that the disilicide of transition
* Corresponding author. Tel.: +81 0 29 859 2223; fax: +81 0 29 859 2401; e-mail: [email protected]
metals is a potential effective additive for ZrB2 ceramics. Therefore, it is necessary for developing low temperature pressureless sintering of ZrB2 ceramics to explore a new disilicide additive of transition metals. In the present study, ZrB2-based composites with ZrSi2 were pressureless-sintered at 1600 °C and 1650 °C. The thermal and electrical properties in the composites were investigated at room temperature by a nanoflash technique and a current–voltage method, respectively. Also, the effects of ZrSi2 amount on the densification, thermal, and electrical conductivities were examined. The starting powders used in this study were: ZrB2 powder (Grade F, Japan New Metals, Tokyo), average particle size 2.1 lm, ZrSi2 powder (Grade F, Japan New Metals), average particle size 2.5 lm. The ZrB2 powder contains 1.6 wt.% B2O3, and the ZrSi2 powder contains 1.4 wt.% SiO2. In order to examine effects of ZrSi2 amount on the densification and properties, four series of ZrB2–ZrSi2 compositions were prepared in this study. Hereafter, four series of ZrB2–ZrSi2 compositions are denoted as ZSZ-1, ZSZ-2, ZSZ-3, and ZSZ-4, respectively. The detailed compositions are shown in Table 1. The powder mixtures were wet-mixed using SiC milling media and ethanol for 24 h, and the resulting slurries were then dried. Before sintering, the dried mixtures were sieved through a metallic sieve with –60-mesh screen size. Pellets were prepared by uniaxial pressing, following by cold isostatic pressing under 350 MPa. Then, the
1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.11.019
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S.-Q. Guo et al. / Scripta Materialia 58 (2008) 579–582
Table 1. Compositions, densities, shear modulus, Young’s modulus, and Poisson’s ratio of the ZrB2–ZrSi2 composites pressureless-sintered at 1650 °C for 60 min
ZSZ-1 ZSZ-2 ZSZ-3 ZSZ-4
Compositions (vol.%) ZrB2
ZrSi2
90 80 70 60
10 20 30 40
Theoretical density (g/cm3)
5.97 5.85 5.73 5.61
True density (g/cm3)
5.77 5.82 5.71 5.60
pellets were pressureless-sintered (FVHP-1-3, Fuji Electric Co. Ltd., Tokyo, Japan) in a graphite crucible at 1600 °C and 1650 °C for 60 min under vacuum of 7.0 103 Pa. After loading the pellets into the crucible, the furnace was heated at a heating rate of 30 °C min1 to the sintering temperature. The resulting pressureless-sintered sample was approximately 10 mm in diameter and 3 mm in thickness. The final densities, were measured by the Archimedes method with distilled water as medium. The elastic moduli measurements of the composites were determined using ultrasonic equipment. The details of measurements and calculations were reported elsewhere [10]. The obtained densities, shear modulus, G, Young’s modulus, E, and Poisson’s ratio, m, of the composite were also summarized in Table 1. The shear modulus and Young’s modulus lowered with increasing ZrSi2 amount because of the lower moduli of ZrSi2 phase than those of ZrB2 one. One exception was a lower shear modulus and Young’s modulus for the ZSZ-1 than those of the ZSZ-2 and ZSZ-3, as a result of the presence of pores (4 vol.%) because the porosity led to the decreases of both the moduli [10]. X-ray diffraction (XRD) was used for crystalline phase identification of the composites. Microstructure of the resulting composites was observed by field emission scanning electron microscopy (FE-SEM). The thermal diffusivity, a, of the composites was measured on a disk-shaped specimen with a diameter of 10 mm and thickness of 2 mm using the nanoflash technique (LFA447/2-4 N, Nanoflash, NETZSCH-Geratebau GmbH, Postfach, Germany). Prior to the measurements, the samples were coated with a colloidal graphite spray in order to enhance the absorption of the xenon light pulse energy and the emission of IR radiation to the temperature detector. Also, the heat capacity, Cp, was measured with alumina as the reference material. Subsequently, the thermal conductivity of the composites, km, is determined from thermal diffusivity, a, heat capacity, Cp, and density, q, of the composites according to the following equation [11]: k m ¼ qC p a
ð1Þ
The calculated values of the thermal conductivity were corrected for the porosity of the composites by the equation reported elsewhere [12]. Moreover, the electrical conductivity measurements of the composites were performed using the four-wire probe at room temperature. A powder supply (Model: 6220, Keithley, Cleveland, Ohio, USA) and digital multimeter (Model: 2182 Nanovoltmeter, Keithley) were used to measure the IV characteristics of the samples.
Relative density (%TD)
Elastic properties
95.7 99.5 99.7 99.9
G (GPa)
E (GPa)
m
179 192 193 164
405 443 449 383
0.13 0.15 0.17 0.17
In Figure 1, the relative densities of the four compositions ZrB2 materials pressureless-sintered at 1600 °C and 1650 °C for 60 min are presented. At 1600 °C, the relative density strongly depended on amount of ZrSi2, and the density increased with ZrSi2 addition. The relative density exceeding 90% was obtained for an additive amount equal to or greater than 30 vol.%. In particular, for 40 vol.% ZrSi2-containing ZrB2 composition (ZSZ4), the density of about 97.1% was obtained. At 1650 °C, the density exceeding 99% was obtained for ZSZ-2, ZSZ-3 and ZSZ-4 powders regardless of ZrSi2 amount. This means that the fully dense ZrB2-based composites with ZrSi2 may be pressureless-sintered at 1650 °C for an additive amount equal to or greater than 20 vol.%. Even for ZSZ-1 powder, the high relative density of 95.7% was obtained. It is evidence that the sinterability of ZrB2 ceramics is remarkably improved by ZrSi2 addition. This indicated that ZrSi2 is an effective additive for lowering the sintering temperature of ZrB2 ceramics. It is common for SiO2 and B2O3 films to be present on the surfaces of the starting ZrSi2 and ZrB2 powder, respectively. It is known that the SiO2 reacts with B2O3 to produce an intergranular liquid phase at a high temperature (below 1500 °C) [13]. Improvement of densification due to the presence of intergranular liquid phase is documented in the literature. Sciti et al. [14] showed that the addition of MoSi2 significantly improved densification of HfB2 powders, as a result of the presence of intergranular liquid phase. They concluded that the presence of intergranular liquid phase favors the process of grain rearrangement as well as improves the packing density of particles, and removes the oxide species from the surface of HfB2 particles. Similar causes are expected for the ZrB2-based composites with ZrSi2 additive investigated in this study.
105 t=60 min 100 Relative Density (%)
Materials
95 90 85 80
1600°C 1650°C
75 70
5
10
15
20
25
30
35
40
45
Amount of ZrSi2 (vol.%)
Figure 1. Plots of relative density measured for the pressurelesssintered ZrB2–ZrSi2 composites as a function of ZrSi2 amount.
S.-Q. Guo et al. / Scripta Materialia 58 (2008) 579–582
Intensity (a.u.)
ZrB 2
ZrSi 2
SiC
ZrO 2
(d)
(c) (b) (a)
20
30
40
50 60 2θ (degree)
70
80
Figure 2. X-ray diffraction patterns of the ZrB2–ZrSi2 composites pressureless-sintered at 1650 °C for 60 min: (a) ZSZ-1, (b) ZSZ-2, (c) ZSZ-3, and (d) ZSZ-4.
In Figure 2, X-ray diffraction patterns for the pressureless-sintered ZrB2–ZrSi2 composites are presented. From this figure, it is found that the ZrB2 and ZrSi2 phases are primary crystalline phases and the trace amount of ZrO2 and SiC phases are also present for all the compositions. The trace of ZrO2 should be attributed to both oxygen contamination of the starting ZrB2 powder and to oxygen take-up during the milling procedure. The presence of SiC phase should be associated with C contamination of the starting ZrSi2 powder as well as with SiC contamination of media during the milling procedure. Microstructure of the ZrB2–ZrSi2 composites is observed under backscattered electron FE-SEM imaging; typical examples are shown in Figure 3. From these images, it is found that the equiaxed ZrB2 grains (brighter contrast) were homogeneous in the microstructure of the composites. In contrast, the ZrSi2 grains (dark contrast) were inhomogeneous and had a bimodal grain size distribution. Some ZrSi2 grains were very fine and they presented at ZrB2 grain boundaries and/or within ZrB2 grains (indicated by arrows in Fig. 3b). Apparently, the fine ZrSi2 particulars result from the mixing process of powders. This means that the mixing process is effective in reducing the size of
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ZrSi2 particles. Some ZrSi2 grains were considerably coarse, and they presented at multiple grains junctions (indicated by arrows in Fig. 3c). The ZrSi2 grains were randomly and individually present in the isolated locations. For ZSZ-4 composition, however, the ZrSi2 agglomerate was observed (indicated by arrows in Fig. 3d). In addition, some pores were observed for ZSZ-1 composition (indicated by arrows in Fig. 3a), while only a little of pores were observed for ZSZ-2, ZSZ-3 and ZSZ-4 compositions. The measured heat capacities, thermal diffusivities, and thermal conductivities of the pressureless-sintered ZrB2–ZrSi2 composites are summarized in Table 2. Although the heat capacities lowered with ZrSi2 amount, this effect is considerably weak. On the other hand, the thermal diffusivity of the composites was almost the constant between 10 vol.% and 20 vol.% ZrSi2, and the diffusivity then decreased with increase of ZrSi2 amount. In particular, for ZSZ-4 composition, a significant decrease was observed. Furthermore, the decrease of the thermal diffusivity due to ZrSi2 addition is larger than that of the heat capacity. This means that the effect of ZrSi2 was larger for the thermal diffusivity than for the heat capacity. The obtained thermal conductivity of the composites significantly lowered with ZrSi2 amount, as a result of the lowered heat capacity as well as of the lowered thermal diffusivity. It is known that the thermal conductivity of the composites depended on the thermal conductivity of the components and the thermal resistance of the grain boundaries. The thermal conductivity of ZrB2 is higher than that of the ZrSi2 [15]. This implies that ZrSi2 additions should be lowered the thermal conductivity of ZrB2–ZrSi2 composites. These effects are closely linked to the amount of ZrSi2 because it affects the heat flow resistance through the components and the grain boundaries. In the studied ZrB2–ZrSi2 composites, the thermal conductivity significantly decreased with ZrSi2 amount (Table 2). This suggests that thermal conductivity of the composites strongly depended on the amount of ZrSi2. The measurements of thermal properties showed
Figure 3. Typical examples of backscattered electron FE-SEM images for the ZrB2–ZrSi2 composites pressureless-sintered at 1650 °C for 60 min: (a) ZSZ-1, (b) ZSZ-2, (c) ZSZ-3, and (d) ZSZ-4.
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Table 2. Thermal properties measured at room temperature in the ZrB2–ZrSi2 composites pressureless-sintered at 1650 °C for 60 min Materials
Heat capacity Cp (J (g K)1)
Thermal diffusivity a (mm2/s)
Measured thermal conductivity km (W(m K)1)
Corrected thermal conductivity kc (W(m K)1)
ZSZ-1 ZSZ-2 ZSZ-3 ZSZ-4
0.56 ± 0.01 0.53 ± 0.01 0.52 ± 0.01 0.50 ± 0.01
30.75 ± 0.31 30.96 ± 0.22 29.01 ± 0.21 26.43 ± 0.09
98.32 ± 0.89 95.83 ± 0.62 86.29 ± 0.58 74.01 ± 0.27
107.16 ± 0.89 96.79 ± 0.62 86.81 ± 0.58 74.16 ± 0.27
Table 3. Electrical properties measured at room temperature in the ZrB2–ZrSi2 composites pressureless-sintered at 1650 °C for 60 min Materials
Electrical resistivity R (X cm)
Electrical conductivity r (X cm)1
ZSZ-1 ZSZ-2 ZSZ-3 ZSZ-4
9.33 106 8.64 106 8.39 106 10.40 106
1.07 105 1.16 105 1.19 105 0.96 105
that the presence of ZrSi2 led to an increase of the heat flow resistance through the components and the grain boundaries (Table 2). The resistance of heat flow increased with increasing ZrSi2, as a result of the reduction of high conductivity ZrB2-phase and the increase of ZrB2/ZrSi2 and/or ZrSi2/ZrSi2 grain boundaries. Thus, accompanying the increase of ZrSi2, the lowered thermal conductivity probably resulted from the high thermal transport resistance of ZrSi2 phase as well as from a higher intergranular thermal resistance at the ZrB2/ ZrSi2 and/or ZrSi2/ZrSi2 grain boundaries than that at the ZrB2/ZrB2 grain boundary. The measured electrical resistivities and conductivities of the pressureless-sintered ZrB2–ZrSi2 composites are summarized in Table 3. It is found that the electrical conductivity of the composites was almost constant for an additive amount equal to or lower 30 vol.% and the conductivity lowered for 40 vol.% ZrSi2-containing ZrB2 ceramic (ZSZ-4). One exception is the low electrical conductivity for the additive amount of 10 vol.% (ZSZ-1) because of the presence of pores. This shows that the ZrSi2 addition has no affect on the electrical conductivity of the composites when the ZrSi2 amount was equal to or lower than 30 vol.% although the conductivity of ZrSi2 is lower that of ZrB2 [15]. This means that the electrical conductivity is insensitive to added ZrSi2 amount for an additive amount equal to or lower than 30 vol.%. In the studied composites, the ZrSi2 grains randomly and individually presented between the ZrB2 grains (Fig. 3). Thus, the ZrB2 grains still remained a closely contact each other for an additive amount equal to or lower than 30 vol.%. This means that the ZrB2 grains with a higher electrical conductivity in the composites formed a network-like structure for providing an electrical path with lower resistance. However, this structure is insufficient for retention of a high thermal conductivity. This seems to suggest that the interfacial resistance at grain boundaries is larger for
heat flow than for electron migration. For ZSZ-4 composition, however, the electrical conductivity of the composite decreased because of the presence of a large amount of ZrSi2 phase and the ZrSi2 particulars agglomerates. This suggests that 30 vol.% ZrSi2 is critical for the high electrical conductivity of ZrB2 ceramic to remain. In summary, the near-fully dense ZrB2-based composites with ZrSi2 additive were pressureless-sintered at 1650 °C for 60 min in vacuum for an additive amount equal to or greater than 20 vol.%. The relative densities exceeding 99% was obtained for these powders. The thermal conductivities of ZrB2–ZrSi2 composites lowered with ZrSi2 amount, and they were in the range 74.16 and 107.16 W (m K)1. On the other hand, the electrical conductivities of ZrB2–ZrSi2 composites were almost the constant for an additive amount equal to or lower than 30 vol.%. However, the conductivity significantly lowered as the ZrSi2 amount was 40 vol.%. The measured electrical conductivities were in the range 0.96 105 to 1.19 105 (X cm)1. [1] K. Kuwabara, Bull. Ceram. Soc. Jpn. 37 (2002) 267. [2] K. Upadhya, J.-M. Yang, W.P. Hoffmann, Am. Ceram. Soc. Bull. 76 (1997) 51. [3] M. Pastor, Metallic Borides: Preparation of Solid Bodies. Sintering Methods and Properties of Solid Bodies, in: V.I. Matkovich (Ed.), Boron and Refractory Borides, Springer, New York, 1977, pp. 457–493. [4] F. Monteverde, A. Bellosi, Scripta Mater. 46 (2002) 223. [5] F. Monteverde, A. Bellosi, Solid State Sci. 7 (2005) 622. [6] F. Monteverde, A. Bellosi, Adv. Eng. Mater. 5 (2003) 508. [7] A. Bellosi, F. Monteverde, D. Sciti, Int. J. Appl. Ceram. Technol. 3 (2006) 32. [8] S.Q. Guo, T. Nishimura, Y. Kagawa, H. Tanaka, J. Am. Ceram. Soc. 90 (2007) 2255. [9] D. Sciti, S. Guicciardi, A. Bellosi, G. Pezzotti, J. Am. Ceram. Soc. 89 (2006) 2320. [10] S.Q. Guo, N. Hirosaki, Y. Yamamoto, T. Nishimura, M. Mitomo, J. Euro. Ceram. Soc. 23 (2003) 537. [11] W.J. Parker, W.J. Jenkins, C.P. Butler, G.L. Abbott, J. App. Phys. 32 (1961) 1679. [12] W.D. Kingery, H.K. Bowen, D.R. Uhlman, Introduction to Ceramics, Eiley, New York, 1976, p. 636. [13] T.J. Rockett, W.R. Foster, J. Am. Ceram. Soc. 48 (1965) 75. [14] D. Sciti, L. Silvestroni, A. Bellosi, J. Mater. Res. 21 (2006) 1460. [15] Manufactures data, Japan New Metals Corporation Ltd., Tokyo, Japan, (http://www.jnm.co.jp).